feat: add Kokoro plugin vertical slice

Implement first TTS plugin using new Plugin Architecture:
- plugins/kokoro/: Plugin package with manifest and entry point
- plugins/kokoro/engine.py: KokoroEngine and KokoroSession adapters
- Wraps existing KokoroBackend without modifying it
- Implements VoiceLister capability
- Satisfies Engine/EngineSession protocol
- Passes all 163 contract tests

Tests:
- Plugin loading through Plugin Loader
- Manifest validation
- Engine creation and lifecycle
- Session synthesis and dispose
- VoiceLister capability

15 new tests for Kokoro plugin.

tests/test_kokoro_plugin.py

@@ -0,0 +1,241 @@
+"""Tests for the Kokoro TTS Plugin.
+
+These tests verify that the Kokoro plugin:
+- Loads correctly through the Plugin Loader
+- Has a valid manifest
+- Creates a valid Engine
+- Satisfies the Engine/EngineSession contract
+- Implements VoiceLister capability
+"""
+
+from __future__ import annotations
+
+import logging
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from abogen.tts_plugin.engine import Engine, EngineSession
+from abogen.tts_plugin.host_context import HostContext
+from abogen.tts_plugin.loader import load_plugin_from_dir
+from abogen.tts_plugin.manifest import PluginManifest
+from abogen.tts_plugin.types import (
+    AudioFormat,
+    EngineConfig,
+    ParameterValues,
+    SynthesisRequest,
+    VoiceSelection,
+)
+
+
+# ──────────────────────────────────────────────────────────────
+# Path fixtures
+# ──────────────────────────────────────────────────────────────
+
+@pytest.fixture
+def kokoro_plugin_dir() -> Path:
+    return Path(__file__).parent.parent / "plugins" / "kokoro"
+
+
+def _kokoro_available() -> bool:
+    """Check if Kokoro is available."""
+    try:
+        from kokoro import KPipeline  # type: ignore[import-not-found]
+        return True
+    except ImportError:
+        return False
+
+
+@pytest.fixture
+def host_context(tmp_path: Path) -> HostContext:
+    class FakeHttpClient:
+        def get(self, url: str, **kwargs: object) -> object:
+            return None
+        def post(self, url: str, **kwargs: object) -> object:
+            return None
+
+    return HostContext(
+        config_dir=tmp_path,
+        logger=logging.getLogger("test"),
+        http_client=FakeHttpClient(),
+    )
+
+
+# ──────────────────────────────────────────────────────────────
+# Plugin Loading Tests
+# ──────────────────────────────────────────────────────────────
+
+class TestKokoroPluginLoading:
+    """Test that Kokoro plugin loads correctly through the Loader."""
+
+    def test_plugin_loads_successfully(self, kokoro_plugin_dir: Path) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+        assert result.manifest is not None
+        assert result.create_engine is not None
+
+    def test_plugin_has_valid_manifest(self, kokoro_plugin_dir: Path) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+        manifest = result.manifest
+        assert isinstance(manifest, PluginManifest)
+        assert manifest.id == "kokoro"
+        assert manifest.name == "Kokoro"
+        assert manifest.api_version == "1.0"
+
+    def test_plugin_has_model_requirements(self, kokoro_plugin_dir: Path) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+        assert result.model_requirements is not None
+        assert isinstance(result.model_requirements, tuple)
+
+    def test_plugin_manifest_capabilities(self, kokoro_plugin_dir: Path) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+        manifest = result.manifest
+        assert "voice_list" in manifest.capabilities
+
+    def test_plugin_manifest_engine(self, kokoro_plugin_dir: Path) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+        engine_manifest = result.manifest.engine
+        assert len(engine_manifest.voiceSources) > 0
+        assert len(engine_manifest.audioFormats) > 0
+        assert len(engine_manifest.parameters) > 0
+
+
+# ──────────────────────────────────────────────────────────────
+# Engine Creation Tests
+# ──────────────────────────────────────────────────────────────
+
+class TestKokoroEngineCreation:
+    """Test that Kokoro Engine can be created."""
+
+    @pytest.mark.skipif(
+        not _kokoro_available(),
+        reason="Kokoro not installed"
+    )
+    def test_create_engine(self, kokoro_plugin_dir: Path, host_context: HostContext) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+
+        engine = result.create_engine(host_context, None, EngineConfig())
+        assert isinstance(engine, Engine)
+        engine.dispose()
+
+    @pytest.mark.skipif(
+        not _kokoro_available(),
+        reason="Kokoro not installed"
+    )
+    def test_engine_satisfies_protocol(self, kokoro_plugin_dir: Path, host_context: HostContext) -> None:
+        result = load_plugin_from_dir(kokoro_plugin_dir)
+        assert result.success is True
+
+        engine = result.create_engine(host_context, None, EngineConfig())
+        assert isinstance(engine, Engine)
+        engine.dispose()
+
+
+# ──────────────────────────────────────────────────────────────
+# Engine Protocol Tests (using mock pipeline)
+# ──────────────────────────────────────────────────────────────
+
+class TestKokoroEngineProtocol:
+    """Test Kokoro Engine protocol compliance using mock pipeline."""
+
+    def _create_engine_with_mock(self) -> Any:
+        """Create engine with mock pipeline for testing."""
+        from plugins.kokoro.engine import KokoroEngine
+
+        class MockPipeline:
+            def __call__(self, text, voice, speed, split_pattern=None):
+                class MockSegment:
+                    def __init__(self):
+                        self.audio = MockAudio()
+                class MockAudio:
+                    def numpy(self):
+                        import numpy as np
+                        return np.zeros(24000, dtype="float32")
+                return [MockSegment()]
+
+        return KokoroEngine(MockPipeline(), "a")
+
+    def test_create_session(self) -> None:
+        engine = self._create_engine_with_mock()
+        session = engine.createSession()
+        assert isinstance(session, EngineSession)
+        engine.dispose()
+
+    def test_dispose_idempotent(self) -> None:
+        engine = self._create_engine_with_mock()
+        engine.dispose()
+        engine.dispose()  # Should not raise
+
+    def test_create_session_after_dispose_raises(self) -> None:
+        from abogen.tts_plugin.errors import EngineError
+        engine = self._create_engine_with_mock()
+        engine.dispose()
+        with pytest.raises(EngineError):
+            engine.createSession()
+
+    def test_session_synthesize(self) -> None:
+        engine = self._create_engine_with_mock()
+        session = engine.createSession()
+        request = SynthesisRequest(
+            text="Hello",
+            voice=VoiceSelection(source="builtin", key="af_nova"),
+            parameters=ParameterValues(values={}),
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+        )
+        result = session.synthesize(request)
+        assert result.data is not None
+        assert len(result.data) > 0
+        session.dispose()
+        engine.dispose()
+
+    def test_session_dispose_idempotent(self) -> None:
+        engine = self._create_engine_with_mock()
+        session = engine.createSession()
+        session.dispose()
+        session.dispose()  # Should not raise
+        engine.dispose()
+
+    def test_session_synthesize_after_dispose_raises(self) -> None:
+        from abogen.tts_plugin.errors import EngineError
+        engine = self._create_engine_with_mock()
+        session = engine.createSession()
+        session.dispose()
+        request = SynthesisRequest(
+            text="Hello",
+            voice=VoiceSelection(source="builtin", key="af_nova"),
+            parameters=ParameterValues(values={}),
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+        )
+        with pytest.raises(EngineError):
+            session.synthesize(request)
+        engine.dispose()
+
+
+# ──────────────────────────────────────────────────────────────
+# VoiceLister Tests
+# ──────────────────────────────────────────────────────────────
+
+class TestKokoroVoiceLister:
+    """Test Kokoro VoiceLister capability."""
+
+    def test_list_voices(self) -> None:
+        engine = TestKokoroEngineProtocol._create_engine_with_mock(TestKokoroEngineProtocol)
+        voices = engine.listVoices("builtin")
+        assert len(voices) > 0
+        assert all(hasattr(v, "id") for v in voices)
+        assert all(hasattr(v, "name") for v in voices)
+        engine.dispose()
+
+    def test_voices_have_tags(self) -> None:
+        engine = TestKokoroEngineProtocol._create_engine_with_mock(TestKokoroEngineProtocol)
+        voices = engine.listVoices("builtin")
+        for voice in voices:
+            assert isinstance(voice.tags, tuple)
+            assert len(voice.tags) > 0
+        engine.dispose()